DC Servomotor
DC servomotor is an ordinary dc motor. But separately excited dc motor. The DC servo motors are further classified into two types :
- Armature controlled type and
- Field controlled type.
Field Controlled DC Servomotor :
Construction of DC Servomotor :
The construction of field controlled DC servo motor as shown in Figure A.
The signal from the servo amplifier is applied to the field winding.
Speed control of DC Servomotor :
- The armature is connected to the constant current source.
- The field current is controlled by the voltage from servo amplifier. This in turn controls the flux.
- The voltage from the servo amplifier is proportional the error signal produced in the control system in which the servomotor is being used.
- This amplified error signal obtained from the servo amplifier is used for controlling the field current.
- The response of the dc servomotor to any change in the servo amplifier output is explained as follows :
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| Figure A |
Conclusion : Thus the speed of field controlled DC servomotor is varied by varying its field current.
Armature Controlled DC Servomotor :
Speed control :
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| Figure B |
- The connections for the armature controlled DC Servomotor are as shown in Figure B. The constant current source is now applied to the field winding hence remains constant.
- Whereas the servo amplifier output is now connected to the armature so that the armature current is dependent on the error produced in the system in which the Servomotor is being used.
- The response of the armature controlled dc servomotor to any change in the servo amplifier output is explained as follows :
- As the error changes the voltage Va from the servo amplifier also changes.
- Hence the armature current changes.
- So the torque produced by the motor changes.
- The motor will respond to change in torque in such a way that the error is minimized.
Conclusion :
- The speed of an armature controlled DC servomotor is varied by varying the voltage applied across the armature winding of the motor.